The high-definition multimedia interface (HDMI™) is a licensable compact audio/video connector interface for transmitting uncompressed digital streams. The HDMI connects digital audio/video (or multimedia) sources (e.g., a set-top box, a DVD player, a personal computer, a video game console, etc.) to a compatible digital audio device and/or video monitor such as a digital television. In contrast to consumer analog standards, the HDMI enforces digital rights management (DRM) on transmitted media.
FIG. 1 shows an exemplary diagram for connecting a multimedia source device 110 to a multimedia sink device 120 through a HDMI link 100. A multimedia source device 110 transmits high speed data using transition minimized differential signaling (TMDS®) characters. The TMDS characters encapsulate video, audio, and auxiliary data and are carried over three TMDS channels 130-1, 130-2, and 130-3. A multimedia sink device 120 receives the TMDS characters and converts them into digital video streams and control codes encoded in the auxiliary data. The control codes include ESS, HSYNC and VSYNC signals. In addition, configuration, system-level control, management, and status information is exchanged between the multimedia source device 110 and the multimedia sink device 120. The system-level control signals include display data channel (DDC) and consumer electronics control (CEO) which are transmitted over channels: SCL 150, SDA 160, and CEO 170.
A clock, typically running at a video pixel rate, is transmitted on a clock channel 140 and is used by the multimedia sink device 120 as a frequency reference for data recovery on the three TMDS channels 130. The TMDS characters are transferred, over the TMDS channels 130, at a rate synchronized with the video pixel rate transported over the clock channel. A pixel rate determines the number of pixels transmitted per second. The transmission rate of TMDS characters is determined by the HDMI standard, for example, in the HDMI specification version 1.4, the data rate is 250 Mb/Sec up to 3.4 Gb/Sec.
The HDMI cable 100 provides a connectivity means that is DC-coupled, thus a capacitor is not used in order to connect the source and sink circuits. As a result, DC current can flow from a source device 110 to a sink device 120, or vice versa. The DC current is a function of the power voltage of the source 110 and sink 120 devices and any resistance between them. Such DC current may be of tens of milliampere (mA) and thus is in lack of any protection such a current may damage the electric circuits of the source and sink device. For example, high DC current level can melt the metal lines in the electric circuits within the integrated circuit (IC).
The problem of high DC current flows is not prominent in present HDMI connectivity, for example, as defined in the HDMI specification, version 1.4a and earlier versions, for the reason that low data transmission rates of signals over the channels 130 and 140 does not deem any termination at the source device 110. For example, for data rates of up to, but less than, 2.25 Gb/Sec, no termination is required. However, with higher data rates, for example, 2.25 Gb/Sec and higher, a termination is recommended to be placed at each line connected to a source port of each of the TMDS channels 130-1 to 130-3 and clock channel 140, to ensure signal integrity and a proper signal transmission. The termination at the source device 110 may cause a DC current path to a ground of a sink device connected to a respective sink port.
This problem is further illustrated in FIG. 2A, where an electric diagram depicting the connectivity of a source port 211 to a sink port 221 through a DC-coupled HDMI connection 230. The source port 211 and sink port 221 may be a port of any TMDS channel or a clock channel of the HDMI. The source port 211 is connected to an output stage 210 of a source device. The output stage 210 includes a termination 212, e.g., in a form of a 50 Ohm resistor. The output stage 210 also includes a power source VSOURCE 213, and devices 214 and 215 which is a current sink device that sets the total current level allowing the setting of the TMDS swing.
The sink port 221 is connected to an input stage 220 of a sink device and also includes a termination 222, e.g., in a form of a 50 Ohm resistor. The input stage 220 also includes a power source VSINK 223 to power the sink device.
In a normal operation of the circuit illustrated in FIG. 2A, current follows from the VSOURCE 213 through the termination 212 and devices 214 and 215 to the ground. When the sink device is on, DC current flows also from the VSINK 223 through the termination 222 and through devices 214 and 215 to the ground at the output stage 210.
However, as illustrated in FIG. 2B, when the sink device is off and the VSINK 223 is grounded. Current follows from the VSOURCE 213 through the termination 212 and devices 214 and 215 to the ground. In addition, DC current flows from the VSOURCE 213 through the termination 212 to the termination 222 at the input stage 220 to the ground. This DC current may damage the source device 210 and the sink device 220 as their current level might be significantly higher than the current level that those devices can handle. For example, if the VSOURCE 213 is a 3.3V power source, then a level of the DC current flows through the termination 212 and through the termination 222, when the VSINK 223 powered off, is 33 mA. In comparison, when the VSINK 223 is on, the level of the DC current that flows through the termination 212 is 12 mA and the level of the DC current that flows through the termination 222 is 12 mA, for a TMDS swing of 1.2v peak-2-peak differentially. Most integrated circuits require operation power voltage of 3.3V and are not designed to handle DC current levels of 33 mA. To support such DC current levels and higher, an IC (that includes, in part, the input stage 220) should be designed with wide metal lines. However, this is an inefficient solution as it increases the size of the IC, and hence its cost and even its performance.
In another scenario, when the sink device is off and the VSINK 223 is floated, a high DC current flows from the VSOURCE 213 through the termination 212 and devices 214 and 215 to the ground. Such DC current may also damage the source device 210 as its DC current level may be significantly higher than the DC current level that the output stage 210 can handle. For example, if the VSOURCE 213 is a 3.3V power source, then a level of the DC current flows through the termination 212, when the VSINK 223 powered off and floated, is 24 mA. In comparison, when the VSINK 223 is on, the level of the DC current that flows through the termination 212 is 12 mA and through termination 222 is 12 mA, for a TMDS swing of 1.2v peak-2-peak differentially.
It should be noted that DC current that flows through the terminations, e.g., terminations 212 and 222, when the sink device is off, is not a spike but rather a steady signal. For example, a TV set (sink device) may be off while a Set-Top box (source device) connected to the TV through a HDMI cable may be powered on. Thus, high DC current levels can flow to the TV set for hours heating and damaging its circuits.
Thus, it would be advantageous to have a solution for protecting HDMI components from DC currents.